Fault detection and clearing control strategy in an islanded microgrid with converter-interfaced sources

A significant factor for properly designing the protection scheme of a microgrid is the fault current contribution from the converter-interfaced distributed energy resources (CI-DERs). The fault current direction and magnitude is difficult to be defined, since the currents injected by the CI-DERs affect it considerably. The fault situation is even more complicated, considering the case of islanded looped microgrids with conventional protection devices. In such microgrids, all protection devices are settled to the same rated current. In order to overcome these issues, this paper proposes a new method for detecting and clearing faults, without utilizing any communication means. The fault identification process is carried out by measuring indirectly the microgrid impedance, while each CI-DER adapts the internal control method and injects a fault current proportional to the sensed impedance. Therefore, the source being closer to the fault injects relatively larger currents enabling the selective coordination of conventional protection devices. In order to enhance even more the selective cooridnation of the protection devices, a time delay is also incorporated in the fault control. The proposed strategy is validated in a looped microgrid, protected by simple overcurrent relays.